Radiographic images are produced by the detection of radiation that is transmitted through the object (e.g. the cargo in a truck or container) being inspected. The density, atomic number and the total amount of material that is present in the object determine how much of the radiation is attenuated and, therefore, the nature and type of radiographic image produced. In addition to determining the absorption of the X-ray, gamma-rays, or neutrons as they travel along their various paths, it is possible to derive information about the characteristics of the material. Conventionally, images are produced using single- or dual-energy X-ray beams to generate attenuation maps and to provide some atomic number information. The identification of areas in the image where high-Z materials, such as special nuclear materials (SNM) are present is of specific security interest related to the detection of certain classes of weapons of mass destruction (WMD). It is desirable for the images to contain additional information to improve the detection of high-Z materials.
Further, the threat of nuclear material and nuclear device smuggling requires a fast and reliable non-intrusive inspection of all types of conveyances, such as containers and cargo at sea and airports or trucks at land ports of entry. Detection of the spontaneous emission of radiation from nuclear material has known limitations, which can be overcome by using active interrogation. Active interrogation typically employs narrow or wide beams of penetrating probes such as neutrons or high-energy X-rays to induce fissions in the nuclear material, if present.
Nuclear material is detected by exposing a container to radiation, such as X-ray radiation or neutrons, and inducing fission by interaction of the radiation with the nuclear material, referred to as photo-fission or neutron fission, respectively. The fission process causes the nuclear material to emit multiple signatures such as Prompt Neutrons, Delayed Neutrons, Prompt gamma rays and Delayed gamma rays. In the past, most systems were designed to detect fission events by detecting the delayed-neutron signature(s) using detector arrays positioned external to the irradiated container. The detection of fission-related delayed neutrons is a very strong indication that nuclear material is present. Delayed neutrons, however, while a unique indicator of the occurrence of fission, are very few in number and of low energy, thereby severely reducing the efficacy of the inspection system, especially for hydrogenous cargo.
In some instances, the sole fission signature measured is that due to the delayed gamma rays. This signature can be highly attenuated in metallic cargos. In these cases, it is much more desirable to detect fission prompt neutrons, which are more abundant and penetrating than delayed neutrons. However, the fission prompt neutrons are produced at virtually the same time as the far more numerous probing radiation incident on the nuclear material; thus resulting in blinding all detectors. Generally, by the time the detectors recover, no prompt-neutron signature exists.
Accordingly, there is need for methods and systems for improving the detection of high-Z materials using high-throughput radiographic means. There is also a need for improved method and systems for confirming the presence of nuclear materials that do not depend solely on one signature, but on multiple signatures where the vulnerabilities of one signature are mitigated by the strength of the others. Similarly, multiple probing radiation types, such as X-rays and neutrons used separately or in simultaneously make the inspection system far more sensitive to concealed nuclear material over the wide range of cargo type encountered in commerce.